1. Field of the Invention
This invention relates to a magnetic recording medium, and more particularly to a non-binder type magnetic recording medium useful as a video recording tape and the like.
2. Description of the Prior Art
As a magnetic recording medium, there have been in wide use coating type magnetic recording media which are formed by applying a dispersion of powdered magnetic material in an organic binder to a non-magnetic substrate and drying it. The coating type magnetic recording media are not suitable for high density recording since, as the magnetic material, metal oxides the saturation magnetization of which is smaller than that of ferromagnetic metals, are used. Further, a coating type magnetic recording medium is disadvantageous in that the manufacturing process is complicated and large scale equipment is required for recovery of solvents and prevention of environmental pollution.
In response to strong demand for high density recording, there have been developed non-binder type magnetic recording media having, as the magnetic recording layer, a ferromagnetic film formed by a vapor deposition process such as vacuum deposition, sputtering, ion plating or the like, or a plating process such as electroplating or electroless plating. The non-binder type magnetic recording medium is advantageous over the coating type magnetic recording medium in that the recording layer can be formed of ferromagnetic metal having a saturation magnetization larger than that of metal oxide used for forming the recording layer of the coating type recording medium and can be formed without non-magnetic material such as the binder being included in the recording layer, and accordingly the recording layer of the non-binder type magnetic recording medium has a higher magnetic coercivity and is thinner compared with the recording layer of the coating type magnetic recording medium. Further, the manufacturing process of the non-binder type magnetic recording medium is relatively simple and is free from environmental pollution due to organic solvents. In particular, the recording layer of the non-binder type recording medium can be thinner than that of the coating type recording medium by a factor of ten, therefore it has been attracting attention as a recording medium for high density magnetic recording.
However, the magnetic film (recording layer) of the non-binder type magnetic recording medium is apt to corrode and is inferior to the recording layer of the coating type magnetic recording medium in resistance to weathering and corrosion prevention properties. In particular, when the recording layer is being contacted by a magnetic head during recording/reproduction, any slight corrosion on the surface of the recording layer can cause clogging with the magnetic head, which can result in damage to the recording medium and the magnetic head. When corrosion of the recording layer increase, the recording layer is apt to peel off, causing loss of recorded information.
Further, there is a problem with durability of the non-binder type magnetic recording medium. That is, the non-binder type magnetic recording medium is inferior to the coating type magnetic recording medium in still-mode durability as used in a video tape recorder, and therefore there has been strong demand for improvement thereof.
There have been proposed various methods of improving the resistance to weathering and durability of the non-binder type magnetic recording medium. For example, it is proposed in Japanese Unexamined Patent Publication No. 50(1975)-33806 to surface-nitride the non-binder type magnetic recording medium by ion plating process. In Japanese Unexamined Patent Publication No. 53(1978)-30304, there is disclosed a method in which a silicon nitride film is formed on the surface of the non-binder type magnetic recording medium by sputtering. In Japanese Unexamined Patent Publication No. 53(1978)-85403 is disclosed a method in which a magnetic film is exposed to discharge in a nitrogen atmosphere to form a non-magnetic surface layer. Further, in Japanese Unexamined Patent Publication No. 54(1979)-143111, there is disclosed a method in which a metal nitride film is formed on a magnetic metal film. However, none of these methods are satisfactory in that the protective layer formed by the various methods described above must be large in thickness in order to achieve sufficient resistance to weathering and sufficient durability. When the protective layer is large in thickness, the electromagnetic transduction properties are significantly deteriorated and the advantage of the non-binder type magnetic recording medium is lost. A non-binder type magnetic recording medium having a magnetic film of iron nitride and iron, or of iron nitride as disclosed in European Pat. No. 8328 and Japanese Unexamined Patent Publication No. 59(1984)-87809 exhibits superior resistance to weathering, but is lacking in magnetic properties and durability.